1. Field of the Invention
This invention relates to a method for forming a radiation image and a method for quantitatively analyzing bone calcium, wherein the method for forming a radiation image is used. This invention particularly relates to a method for forming a radiation image by using stimulable phosphor sheets and an energy subtraction processing technique.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays, such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as a human body, is recorded on a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet). The stimulable phosphor sheet, on which the radiation image has been stored, is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, when it is exposed to the stimulating rays, is photoelectrically detected and converted into an electric image signal. The electric image signal is then processed, and the processed image signal is then used during the reproduction of a visible image which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness. The visible image finally obtained may be reproduced in the form of a hard copy or may be displayed on a display device, such as a cathode ray tube (CRT) display device. In the radiation image recording and reproducing systems, the stimulable phosphor sheet is used to store the radiation image temporarily so that a final visible image can be reproduced therefrom on a final recording medium. For the sake of economy, therefore, it is desirable that the stimulable phosphor sheet be used repeatedly.
In order that the stimulable phosphor sheets may be reused as described above, the energy remaining on the stimulable phosphor sheet after it has been scanned with stimulating rays should be erased. For this purpose, the stimulable phosphor sheet may be exposed to light or heat as described in, for example, U.S. Pat. No. 4,400,619 or Japanese Unexamined Patent Publication No. 56(1981)-12599. The stimulable phosphor sheet may then be used again for the recording of a radiation image.
Also, techniques for carrying out subtraction processing on radiation images have heretofore been known. When subtraction processing is to be carried out, at least two radiation images recorded under different conditions are photoelectrically read out, and digital image signals which represent the radiation images are obtained. The image signal components of the digital image signals which represent corresponding picture elements in the radiation images are then subtracted from each other, and a difference signal is thereby obtained which represents the image of a specific structure or part of the object represented by the radiation images. With the subtraction processing method, at least two digital image signals are subtracted from each other in order to obtain a difference signal, and the radiation image of a specific structure can be reproduced from the difference signal.
Basically, subtraction processing is carried out with either the so-called temporal (time difference) subtraction processing method or the so-called energy subtraction processing method. In the former method, in order for the image of a specific structure of an object to be extracted from the image of the whole object, the image signal representing a radiation image obtained without injection of contrast media is subtracted from the image signal representing a radiation image in which the image of the specific structure of the object is enhanced by the injection of contrast media. In the latter method, an object is exposed several times to radiation with different energy distributions, or the energy distribution of the radiation, which has passed through an object, is changed after it has been irradiated onto one of at least two radiation storage means, after which the radiation impinges upon the second storage means. In this manner, at least two radiation images, in which different images of a specific structure are embedded, are obtained. Thereafter, the image signals representing at least two radiation images are weighted appropriately, when necessary, and subjected to a subtraction process in order to extract the image of the specific structure. In general, of the energy subtraction processing method, the method, wherein an object is exposed several times to radiation with different energy distributions, is referred to as the "two-shot energy subtraction processing method." Also, the method, wherein the energy distribution of the radiation, which has passed through an object, is changed after it has been irradiated onto one of at least two radiation storage means, after which the radiation impinges upon the second storage means, is referred to as the "one-shot energy subtraction processing method."
It is advantageous that stimulable phosphor sheets are utilized during energy subtraction processing wherein a subtraction process is carried out on image signals. For example, in the two-shot energy subtraction processing method, a first stimulable phosphor sheet located at the position for image recording is quickly exchanged with a second stimulable phosphor sheet and, at the same time, the level of the tube voltage of an X-ray tube is quickly changed over such that radiation having a high energy level and radiation having a low energy level may be sequentially irradiated to an object. In this manner, a radiation image is recorded on one of the stimulable phosphor sheets with the radiation having a high energy level, and a radiation image is recorded on the other stimulable phosphor sheet with the radiation having a low energy level. In the one-shot energy subtraction processing method, for example, at least two stimulable phosphor sheets are placed one upon another with a radiation energy separating filter, such as a copper plate, intervening therebetween. Alternatively, at least two stimulable phosphor sheets having different radiation absorption characteristics are placed one upon another. In this manner, different kinds of radiation images are simultaneously recorded on the stimulable phosphor sheets placed one upon another. The applicant proposed novel energy subtraction processing methods using stimulable phosphor sheets in, for example, U.S. Pat. Nos. 4,855,598 and 4,896,037.
Subtraction processing is extremely effective, particularly for medical diagnosis, and research has continued to develop improved subtraction processing methods. For example, it has been proposed to quantitatively analyze bone calcium by utilizing the energy subtraction processing technique, wherein patterns of soft tissues of an object are erased, and an image representing only the bones of the object is formed. In the quantitative analysis of bone calcium, amounts of calcium in bones of a human body, or the like, are quantitatively determined. Quantitative determination of the amounts of calcium in bones is necessary for preventing fractures of bones. Specifically, by investigating small changes in the amounts of calcium contained in bones, osteoporosis can be found early, and fractures of the bones can be prevented.
In U.S. Pat. No. 5,122,664, the applicant proposed a novel method for quantitatively analyzing bone calcium, wherein energy subtraction processing is employed. Specifically, the applicant proposed a method for quantitatively analyzing bone calcium by carrying out energy subtraction processing wherein each of at least two stimulable phosphor sheets is exposed to one of at least two kinds of radiation, which have different energy distributions and have passed through an object constituted of bones and soft tissues, radiation images of the object are thereby recorded on the stimulable phosphor sheets, each of the stimulable phosphor sheets is thereafter exposed to stimulating rays, each radiation image is photoelectrically detected and converted into a digital image signal made up of a series of image signal components representing each radiation image, the image signal components of the digital image signals thus obtained, which image signal components represent corresponding picture elements in the radiation images, are then subtracted from each other, and a difference signal is thereby obtained which represents the image of only the bones represented by the radiation images. The proposed method for quantitatively analyzing bone calcium comprises the steps of:
i) recording a pattern of a bone calcium reference material, which simulates amounts of bone calcium varying step-wise, together with the pattern of the object when each of the radiation images of the object is recorded on each of the stimulable phosphor sheets, and PA1 ii) quantitatively analyzing bone calcium in the bones by comparing the image density of the patterns of the bones with the image density of the pattern of the bone calcium reference material, both patterns appearing in the image of only the bones (i.e., the bone image). PA1 the method for forming a radiation image comprising the steps of:
With the proposed method for quantitatively analyzing bone calcium, an image signal for compensation for the adverse effects of shading (such as nonuniformity in each stimulable phosphor sheet, nonuniformity in how radiation is irradiated to each stimulable phosphor sheet, and nonuniformity in how the light emitted by each stimulable phosphor sheet is detected) is obtained from the stimulable phosphor sheets exposed respectively to at least two kinds of radiation, which have different energy distributions and have not passed through the object. The image signal for compensation and the image signal representing the radiation image are then subtracted from each other. In this manner, adverse effects of the shading are eliminated such that an accurate analysis of bone calcium can be carried out.
However, in cases where an image is recorded with radiation having a broad energy distribution during energy subtraction processing, the so-called "beam hardening phenomenon" occurs. Specifically, the energy distribution of the radiation, which has passed through the object, shifts to the high energy side as a whole. If the beam hardening phenomenon occurs, the drawbacks will occur in that, for example, in a bone image in which soft tissue patterns have been erased, even if the bone structure is the same, the image density of the bone, which is located at a position at which the thickness of the object is large, becomes lower than the image density of the bone, which is located at a position at which the thickness of the object is small. Therefore, during the quantitative analysis of bone calcium, the problems will occur in that the results of determination of the bone density and the amount of bone calcium are adversely affected by the thickness of soft tissues overlapping upon the bone, and the accuracy of the determination cannot be kept high.